The present invention relates to a method for the damping force adjustment of motor vehicles in dependence on output signals of a transmitter or pick-up arranged at the vehicle body which are processed and which trigger a signal for changing the damping force when exceeding/dropping below a predetermined threshold value.
In a known method of this type (EP No. 01 51 421-A) the output signals of the transmitter are analyzed as regards their frequency. The damping force adjustment takes place in dependence of the preferred low-frequency component. This component is representative for the vibrations or oscillations of the vehicle body. In contrast thereto is a higher frequency component which is representative for the vibrations or oscillations of the vehicle wheel, respectively of the vehicle wheels.
At the outset, the technical measurement expenditures are relatively high with the known method because the evaluation of the transmitter-output signal is possible only with the aid of a relatively costly frequency analysis by means of a series-connected processing device connected in the output thereof. Added thereto is the fact that drift occurrences of the used electronic components have a considerable influence on the measurement result. Provision is made to eliminate this influence by matching the two threshold values determinative for the respective frequency components corresponding to the output signal of the control apparatus. This, however, leads to a further increase of the expenditure with the additional effect that the manner of operation of the method is no longer reproducible.
The present invention is concerned with the task to provide a method of the aforementioned type, by means of which it is possible in a simple manner which can be readily monitored, to undertake the shifting between at least two damping force steps.
The underlying problems are solved according to the present invention in that the changes of the output signals of the transmitter which are measured at equal intervals with respect to time, are added and are determined over a measuring period which is considerably greater than the natural period of the vehicle wheels and which are compared with a threshold value that is dependent on the load. The thus-obtained magnitude will be referred to hereinafter as duration dynamic characteristic value (DDK).
By taking into consideration only the changes with respect to time of the output signals of the transmitter, drift appearances of the electronic components are completely eliminated. A frequency analysis also is not necessary because for carrying out the method only the numerical subtraction of successive measurement values is necessary. The consideration of the load for the adjustment of the threshold value, on the one hand, is possible coupled with low expenditure and therebeyond offers a significant functional advantage. The latter rests on the fact that, for example, at large load, an adjustment of the damping force in the direction "hard" is necessary considerably earlier, i.e., at road conditions at which without load a comfortable, respectively, more comfortable damping force adjustment, is still acceptable.
By taking into consideration the change with respect to time of the output signals of the transmitter, it also becomes possible to differentiate between the different influences on the comfort behavior of a motor vehicle and to take into consideration exclusively the determinative components. The magnitudes influencing the DDK-value only as regards its level are the driving velocity and the road unevenness whereas the load and the respectively adjusted damping force step affect the DDK-value calculated from the signals of the transmitter only in the form of a certain dispersion width. Conclusions can thus be drawn with respect to the excitation condition, which results from the road surface and the driving velocity, exclusively from the level of the DDK-values.
If the calculated DDK-values exceed load-dependent thresholds, the shifting takes place into another damping force step.
As a determination of the load condition is not readily possible from the curve of the DDK-value, an additional sensor is provided for this purpose. For example, in vehicles with a level regulation the sensing of the system pressure offers itself for this purpose.
It may be desirable to undertake the adjustment of the damping force in more than two steps, for example, in three steps. In this case, threshold values for the transition between adjacent damping force-adjusting steps may be load-dependent. The number of the possible steps, however, is not limited so that with a corresponding large number of steps, this leads to a quasi-continuous, respectively, continuous damping force adjustment.
Finally, further parameters determinative for the damping force adjustment can be taken into consideration. They include thereby the outside temperature which has a determinative influence on the viscosity of the oil used customarily in dampers and therewith also on the adjusted damping force. The mentioned threshold values are changed temperature-dependent in order to prevent a shifting into an unnecessarily hard damping force step at low temperatures.
The steering angle, the vehicle cross-acceleration, respectively, a substitute magnitude calculated from the driving velocity and steering angle may be utilized as further parameter for changing the mentioned threshold value.